feat(story-2.3): implement home marker and airspace outline rendering

Adds draw_home_marker() in overlay.py (red cross at projected home
position) and draw_airspace() in airspace.py (GeoJSON Polygon
boundaries in blue, graceful FileNotFoundError handling). Includes
airspace fixture and three tests covering all acceptance criteria.
All 70 tests pass; ruff check and format clean.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/fixtures/airspace_sample.geojson

@@ -1 +1,21 @@
-{"type": "FeatureCollection", "features": []}
+{
+  "type": "FeatureCollection",
+  "features": [
+    {
+      "type": "Feature",
+      "geometry": {
+        "type": "Polygon",
+        "coordinates": [
+          [
+            [-6.5, 53.3],
+            [-5.5, 53.3],
+            [-5.5, 53.7],
+            [-6.5, 53.7],
+            [-6.5, 53.3]
+          ]
+        ]
+      },
+      "properties": {"name": "Test Airspace"}
+    }
+  ]
+}